Chemical Vapor Deposition Growth of Two-Dimensional Monolayer Gallium Sulfide Crystals Using Hydrogen Reduction of Ga2S3.

Two-dimensional gallium sulfide (GaS) crystals are synthesized by a simple and efficient ambient pressure chemical vapor deposition (CVD) method using a single-source precursor of Ga2S3. The synthesized GaS structures involve triangular monolayer domains and multilayer flakes with thickness of 1 and 15 nm, respectively. Regions of continuous films of GaS are also achieved with about 0.7 cm2 uniform coverage. This is achieved by using hydrogen carrier gas and the horizontally placed SiO2/Si substrates. Electron microscopy and spectroscopic measurements are used to characteristic the CVD-grown materials. This provides important insights into novel approaches for enlarging the domain size of GaS crystals and understanding of the growth mechanism using this precursor system.

Introduction

Monolayer and continuous two-dimensional (2D) transition-metal dichalcogenides (TMDs) have attracted great interest because of their unique physical properties and promising future applications.[1−8] The intensively studied TMDs, such as MoS2 and WS2, have dichalcogenide form of MX2; similarly, the post-transition metals of group III and VI elements have the form of MX where M = Ga, In and X = S, Se with monochalcogenide form.[9−14] Two bonded gallium metal atoms are between two layers of sulfur atoms, like a sandwich. The unit cell of gallium sulfide (GaS) is hexagonal crystal structure with a = b = 3.587 Å, c = 15.492 Å, where c is equal to two layers of GaS.[15] The indirect band gap of 2D GaS is about 3.05 eV which lies between the semimetal graphene and insulating hexagonal boron nitride.[16−20] Because of the strong surface effects, the first-principle calculated band gap of GaS nanosheets varies from 2.59 to 1.6 eV, when the number of layers is increased from monolayer to bulk, which makes it a promising material for applications in electronics and opto-electronics.[21,22] Also, 2D GaS has been explored for applications in transistors, photodetectors, energy storage, gas sensing, and application as hydrogen evolution catalysts.[14,17,22−24]

Chemical vapor deposition (CVD) has been one of the most promising methods of producing large-area and high-quality 2D materials, as well as for the synthesis of 2D GaS materials.[25−31] Other synthesis methods include mechanical exfoliation, chemically assisted exfoliation, atomic layer deposition, and screen printing deposition methods. Barron et al. demonstrated CVD-synthesized polycrystalline gallium sulfide thin films using a predesigned single-source precursor of gallium chalcogenide cubane [(t-Bu)GaS]4 in 1992.[32] Zhou et al. synthesized various morphologies of GaS such as nanowires, nanobelts, and hexagonal microplates using mixtures of GaN and Bi2S3 precursors, and the reaction took place at 1170 °C using a vapor–solid method.[33] Kang et al. reported GaS1–Se alloy multilayers and monolayers synthesized on a SiO2/Si substrate (with 1 nm thick Au deposited film) at 950 °C using Ga2S3 and Ga2Se3 as precursors by the chemical vapor transport method.[34]

Dravid et al. mechanically exfoliated a single layer of GaS at room temperature under ambient conditions from a bulk material onto a SiO2/Si substrate, and the typical flakes had dimensions of tens of micrometers.[19] Colmen et al. demonstrated the large quantities production of GaS via liquid exfoliation with lateral sizes and thicknesses of 50–1000 nm and 3–80 layers, respectively.[24] Meng et al. synthesized the amorphous and smooth gallium sulfide material via atomic layer deposition method using hexakis digallium and hydrogen sulfide as precursors in the temperature range of 125–225 °C.[35] Recently, Kalantar-Zadeh et al. developed a screen printing method for the deposition of bilayer 2D GaS thin films, which utilized an atomically thin oxide layer and further sulfurization process to replace the oxygen atoms with sulfur atoms.[18] The controllable production of continuous, large-scale, high-quality GaS films still remains a challenge, and further research is needed to improve the material quality and uniformity of the samples.

In this work, we develop a method for the growth of 2D GaS on SiO2/Si substrates using the ambient pressure CVD method. The growth process of GaS includes the evaporation and reduction of Ga2S3 precursor and using hydrogen carrier gas. Both monolayer and continuous GaS film are achieved by controlling the growth temperature, substrate position, and the carrier gas flow rate. The CVD-grown GaS is characterized by optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), photoluminescence (PL) and Raman spectroscopy, and transmission electron microscopy (TEM).

Results and Discussion

Figure a shows a schematic illustration of the CVD method to grow GaS on SiO2/Si substrates. The chemical reaction takes place inside a 1 in. diameter quartz tube under ambient pressure. The Ga2S3 precursor is placed at the center of the growth furnace. The 2 cm by 2 cm substrate is placed at the downstream of the growth furnace with a distance of 8.5 cm. Figure b represents the profiles of temperature against time employed for the growth of GaS and includes three stages. The whole system is flushed with 500 sccm argon gas for 30 min. After that, the temperature of the precursor was increased from room temperature to 800 °C with a mixture of 70 sccm of hydrogen gas and 150 sccm of argon gas. The reaction is conducted once the temperature is reached for a short period of time with only 70 sccm hydrogen gas flow. At last, fast cooling process is applied with only 150 sccm argon gas flow. By optimizing the reaction parameters, such as the growth time, growth temperature, the amounts of precursors, the positions of the substrate, and the amount of hydrogen gas flow rate, we are able to produce the monocrystalline GaS domains or large-area continuous polycrystalline GaS film and even control the chemical composition between the gallium and sulfur. Two modifications are made in stage II. The first modification involves placing the substrate horizontally which creates a precursor concentration gradient and an inhomogeneity of the precursor feed-stock on the substrate surface. The other modification involves increasing the hydrogen gas flow rate during the growth stage and tuning the Ga2S3 decomposition (the compared SEM images are in the Supporting Information). Figure c shows the deposition pattern of after CVD growth of 2D GaS on SiO2/Si substrate and this formation of U-shaped pattern is because of the variation of Ga2S3 vapor concentration gradient. This U-shaped pattern is observed in the CVD growth of other 2D TMD growth such as WS2 and MoS2.[25,26]Figure d,e are two zoomed-in optical images of selective regions in Figure c; both show the multilayer GaS flakes stacked area and individual triangle-shaped GaS domains with the size of 4 μm, respectively.

Figure 1

Atmosphere pressure CVD synthesis of large-area gallium sulfide (GaS) on the SiO2/Si substrate. (a) Schematic illustration of the CVD setup of the synthesis of Ga2S3 white precursor powder is placed in the middle of the growth furnace. (b) Temperature against time profiles of the substrate with three stages includes heating stage I, maintaining stage II, and the last cooling stage III. (c) Photograph showing the overall after growth morphology of GaS on the 2 cm by 2 cm SiO2/Si substrate. The various contrast difference results from the thickness of the grown GaS materials. Two optical images of two separated regions labeled on the overall photo (c). (d) Thicker region of GaS located near the center indicates that the various shaped crystallized GaS flakes are stacked together. (e) Two individual triangular GaS domains with about 4 μm size.

Several stages are observed during the GaS layered growth. To study the morphology of the CVD-grown gallium sulphide 2D materials, SEM is used to examine the SiO2/Si substrate area by area. Figure a presents a schematic illustration of six selected regions which are numbers labeled, where the SEM images are taken. These six regions clearly reveal the dependence of the shape change on the distance from the Ga2S3 precursor vapor concentration. Figure with position 1 shows the morphology of the center and upstream region where multilayer gallium sulfide flakes are formed. Toward the edge of the substrate at positions 2, 3, 4, isolated triangle GaS domains with the size ranging from about 8 and 2 μm are observed. Positions 4-1 to 4-3 represent the growth morphology of position 4 with more details. Furthermore, at position 4-1, the transition between individual domain and continuous film of GaS is found, half of the region shows complete continuous film and the other half region shows the triangular domains with various sizes. Toward the center of the substrate, a uniform continuous film of GaS is observed, formed by the merger of GaS domains and the size of GaS triangular domains increase from 2 to 6 μm, as shown in 4-2, and 4-3 SEM images, respectively. At the downstream region of the substrate, there is a change in the size of GaS domains. At positions 5 and 6, the GaS domain size is reduced to less than 2 μm and rounded nucleation sites are observed, and eventually the size is too small to identify its shape at the end. It can be seen that, with the increase in the distance between the precursor and the growth locations, GaS domains experience a regular morphology transformation as well as size and shape changes.

Figure 2

SEM morphology study of the U-shaped CVD-grown GaS on SiO2/Si substrates. (a) Schematic of the U-shaped region with number labels. Each number label represents the typical regions and the morphologies are shown in the corresponding SEM images.

The continuous region of CVD grown 2D GaS is further investigated because these qualities such as large-area, uniform, and continuous qualities are vital for future applications. Figure a shows the SEM image of a large continuous GaS region on a SiO2/Si substrate, with the left corner having partially covered regions where the triangle GaS domains are. While complete continuous film coverage of GaS is shown in Figure b with the size of 700 μm. It is observed that the large white particles are first deposited onto the SiO2/Si substrate and then gradually enclosed regions. Next, small triangular domains are nucleated at preferred locations, such as substrates’ edges, scratches, dust particles or rough surfaces.[36] More SEM images are shown in the Supporting Information. Then, the nucleation sites continued to grow and formed boundaries when two or more layered 2D GaS meet, resulting in a partially continuous film. This process eventually leads to domains being coalesced and merged into large-area single-layered GaS continuous film, if sufficient precursor supply and denser nucleation sites are provided. A further zoomed-in SEM image of the GaS film is shown in Figure c, and the small dark regions on the continuous films are secondary layers which are formed during CVD growth.

Figure 3

Surface morphology of monolayer GaS films. (a) SEM image of large-area monolayer GaS film with initial nucleation sites. (b) SEM image of selected uniform region of monolayer GaS film. (c) SEM image of monolayer GaS film with second layer nucleation sites.

The energy-dispersive X-ray (EDX) spectroscopy, AFM, Raman spectroscopy, PL spectroscopy, and TEM are all used to evaluate the CVD-grown GaS in terms of chemical compositional analysis, thickness, film quality, and crystallinity. The typical EDX spot spectra are in Figure a,b, where the five spots EDX results are from GaS flakes and the monolayer GaS film region and shown in the corresponding inserts, respectively. The intensity of Ga and S from GaS flakes is significantly strong than the monolayer GaS. The atomic percentage and ratio of sulfur and gallium elements are obtained from the EDX. Figure c reveals the S/Ga ratio comparison among the CVD-synthesized GaS flakes, monolayer region, and the Ga2S3 precursor. The S/Ga ratios of GaS flakes is close to 1 and is consistent to about 1.2 value, whereas the ratio in monolayer GaS has large variation between 1.9 and 1.4. The ratio of the Ga2S3 precursor is set to 1.5 as a reference. This might be due to the ultrathin property of the material, making it difficult to detect subtle variations using EDX. All this information is summarized in Table S1 in the Supporting Information. In this work, the shape of CVD-synthesized GaS domain is quite consistent with a truncated triangle shape. In previous reports, the shape of CVD-grown GaS domains were hexagonal and sharp triangle shapes, and the hexagon-shaped domain has three Ga- and S-terminated edges, and the triangle-shaped domain can only have either S- or Ga-terminated edges.[33,34] The exact ratio between Ga and S atoms on the substrate influences the energetic stability of Ga- and S-terminated edges. Because a single-source precursor Ga2S3 is used here, and it is reduced by the hydrogen carrier gas, it means that only slightly excessive S is present throughout the whole synthesis process. This explains why the resulted GaS domains are truncated, because of a similar Ga/S ratio in the precursor. If the stoichiometry was such that S was in vast excess, we would expect sharp triangular domains. Similar results are reported in the CVD synthesis of MoS2 and WS2 as well.[26]

Figure 4

EDX analysis of gallium and sulfur from CVD-grown GaS. (a,b) Typical EDX spectra of GaS flakes and the GaS monolayer region, respectively. Both insets show SEM images where the five EDX spot spectra were obtained. The scale bar is 8 μm. (c) Ratio of sulfur to gallium and five positions between CVD-grown GaS flakes and domains. The reference ratio 1.5 is used from the Ga2S3 precursor.

The roughness and thickness of the CVD-grown GaS continuous film, flakes, and domains are characterized by AFM. The AFM measurements in the topological mode for an almost fully merged region with corresponding zoomed-in AFM scans, indicated by dashed green and blue boxes, are shown in Figure a–c. This scanned region is not fully covered on the substrate; it is densely covered by triangular GaS domains, which are close to coalescing into a continuous film. Both the merged grain boundaries and the second nuclei are observed under the AFM measurements and a 0.19 nm root mean squared (rms) is measured, indicated by the green box, shown in Figure c. The corresponding height profile of the GaS continuous film is measured in Figure d and the 1 nm thickness is in agreement with the GaS monolayer flakes on SiO2/Si substrate.[19] The GaS flakes are also measured along the black line indicated in Figure e and the corresponding height profile results in two steps with 25 and 10 nm thickness of the GaS flakes, as shown in Figure f. The individual triangular GaS domain has about 0.6 nm step height, although the measured rms on the after CVD-grown SiO2/Si substrate is 0.48 nm on the same scan; both are shown in Figure g,h, respectively. The rms on the triangle GaS domains closely match the monolayer film, with both measured to have values of ∼0.16 nm.

Figure 5

(a–c) Series of AFM images on same region with different magnifications. The zoomed-in regions are labeled in green and blue dashed boxes. The highlighted green color region has an rms of 0.19 nm. (d) Height profile of a continuous GaS film measured across the black line in (c). (e) AFM image of as-grown GaS flakes. (f) Corresponding height profile of selected GaS flakes measured across the black line in (e). (g) AFM images of two GaS domains. Box 1 has an rms of 0.48 nm on the SiO2/Si substrate, the boxes 2 and 3 have the same rms of 0.16 nm on the GaS domains. (h) Corresponding height profile of the GaS domain measured across the black line in (g).

The as-grown GaS flakes and monolayer domains are further investigated using Raman spectroscopy with a 532 nm excitation to determine the layer number and quality. GaS has three first-order phonon modes in Raman spectroscopy, two out-of-plane A1g1 and A1g2 modes, and one in-plane E2g1 mode. Raman spectra are taken from spot positions on both GaS flakes and domains, as shown in Figure a,b, respectively. The GaS flakes show Raman peaks for modes A1g1 (188 cm–1), A1g2 (360 cm–1) and E2g1 (294 cm–1). The full width at half-maximum of A1g1 and A1g2 mode are 5 and 6 cm–1, respectively. However, the GaS monolayer domain shows A1g1 (179 cm–1), E2g1 (297 cm–1) which is combined with the 303 cm–1 Raman peak from the SiO2/Si substrate. This results in a broadened peak, and the A1g2 is not shown on the spectra. As the thickness of GaS decreases from flakes to monolayer domain, the frequency of the E2g1 mode slightly increased and that of A1g1 mode decreased, and also, the overall intensities are observed to decrease as well. These results are in agreement with previous reports.[17,19,22,37] The spectroscopy measurements of the GaS flakes with 532 nm excitation also show a very broad peak between 550 and 800 nm with centered position at 622 nm. This peak is associated with either indirect transitions or defects in GaS flakes that produce deep trap recombination states.[38] However, the monolayer GaS domain does not show this peak at all. The 532 nm excitation is not sufficient to excite the indirect band gap of GaS, and thus, we do not expect to observe high-efficiency PL from recombination across the indirect gap. These two strong PL peaks observed in both samples and obtained by 523 nm excitation is associated with the SiO2/Si substrates.

Figure 6

Raman spectra and PL measurements of CVD-grown GaS flakes and monolayer domain on the SiO2/Si substrate. (a) Raman spectrum and (c) PL emission spectra of the multilayer stacked GaS flakes region. (b) Raman spectrum and (d) PL emission spectra of monolayer triangle-shaped GaS domain. Both inserts are the corresponding optical images, and the scale bar is 5 μm.

TEM is used to further characterize the CVD as-grown gallium sulfide layered structures after transfer onto TEM grids by a poly(methyl methacrylate)-assisted wet chemical transfer method. Figure a–c shows typical low-magnification and high-resolution TEM (HRTEM) images of thin layered GaS sheet, which is on a lacey carbon TEM grid. Figure a shows the layered structures at a large scale. Figure b,d shows HRTEM images, and they reveal the single-crystalline nature of the CVD-grown GaS flakes. Intensity line profiles are plotted in Figure c, and the measured d-spacing between the neighboring (100) planes is 0.31 nm which is consistent with the values for β-GaS (a = 3.587 Å and c = 15.492 Å).[14,33] The inset in (b) shows corresponding fast Fourier-transform (FFT) power spectrum indicating that GaS sheets are orientated along at the [0001] zone axis and confirm the hexagonal structure, compared with previous report.[34] Further zoomed-in HRTEM images are shown in Figure d, which corresponds to the selected area in Figure b; and the schematic diagram of the top view of the GaS crystal is presented in Figure e. Both help to confirm the crystalline quality of CVD-grown GaS materials and are in good agreement with the mechanical exfoliated prepared GaS nanoflakes.[14]

Figure 7

TEM images of GaS multilayer flake. (a) Low-magnification TEM image of GaS flake shows the layered structure. (b) HRTEM and corresponding FFT image of GaS monolayer region at the zone axis of [0001], indicating the single-crystalline nature. (c) The corresponding line intensity profile is taken form the yellow marked region in (b). It results the (100) planes of GaS are separated by a distance of 0.31 nm. (d) Zoomed-in HRTEM image of GaS flakes corresponding to the selective area from (b). (e) Schematic illustration of the top view GaS crystal from the [0001] zone axis.

Conclusion

In conclusion, we report a simple CVD approach for growing both monolayer and multilayer high-quality GaS films with large domain size on SiO2/Si substrate using hydrogen carrier gas to reduce a single-source precursor of Ga2S3 at atmospheric pressure. The film microstructure, thickness, homogeneity, and quality are characterized by optical microscopy, SEM with EDX, Raman spectra, PL spectra, and TEM. These results show how sensitive the growth of GaS is to minor variations in the CVD parameters and that drastic improvements can be easily achieved by simple optimizations. This approach has shown potential in scale-up and meets safety requirements for large-scale implementation and eventually toward the controllable number of layers CVD growth of 2D GaS.